1. Field of the Invention
The present invention relates to a nuclear gauge for measuring a characteristic of a sheet material, and, more particularly, to a nuclear gauge for measuring a characteristic of a moving sheet material with sheet position compensation and alignment compensation.
2. Description of the Related Art
Nuclear gauges are used for measuring thicknesses of a moving sheet material, in a non-contact manner, by detecting the amount of radiation that passes through the sheet material and/or the dispersion of the radioactive emission by the sheet material. The scattering effect that the sheet material imparts to the radiation beam is often compensated for by shaping the radiation beam in an attempt to achieve insensitivity to gap size variations and flutter of the sheet material as it moves through the gauge. Flutter compensation is typically done with collimators on both the radiation source and the radiation receiver to thereby exclude rays that are deviated by a large angle due to the influence of the sheet material. Radiation source and receiver collimation is adjusted to minimize the sensitivity of the detector to flutter of the sheet material. This method usually works over only a limited range and different measurement ranges require different geometries to compensate for flutter of the sheet material. This method of compensation results in a reduce signal to noise ratio as the detection properties are limited to compensate for the flutter of the sheet material.
Compensation of gap variations is often done by blocking the center of the radiation beam being directed to the detector with a radiation absorbing disk. This disk, which is often called an R2 disk, results in decreased sensitivity to gap size variation but only over a narrow range.
What is needed in the art is a way of compensating for variations in the position of the material sheet between the radiation source and the detector of a nuclear gauge.